IMPORTANCE: The term golden hour was coined to encourage urgency of trauma care. In 2009, Secretary of Defense Robert M. Gates mandated prehospital helicopter transport of critically injured combat casualties in 60 minutes or less. OBJECTIVES: To compare morbidity and mortality outcomes for casualties before vs after the mandate and for those who underwent prehospital helicopter transport in 60 minutes or less vs more than 60 minutes. DESIGN, SETTING, AND PARTICIPANTS: A retrospective descriptive analysis of battlefield data examined 21 089 US military casualties that occurred during the Afghanistan conflict from September 11, 2001, to March 31, 2014. Analysis was conducted from September 1, 2014, to January 21, 2015. MAIN OUTCOMES AND MEASURES: Data for all casualties were analyzed according to whether they occurred before or after the mandate. Detailed data for those who underwent prehospital helicopter transport were analyzed according to whether they occurred before or after the mandate and whether they occurred in 60 minutes or less vs more than 60 minutes. Casualties with minor wounds were excluded. Mortality and morbidity outcomes and treatment capability–related variables were compared. RESULTS: For the total casualty population, the percentage killed in action (16.0% 386 of 2411 vs 9.9% 964 of 9755; P < .001) and the case fatality rate (CFR 13.7 469 of 3429 vs 7.6 1344 of 17 660; P < .001) were higher before vs after the mandate, while the percentage died of wounds (4.1% 83 of 2025 vs 4.3% 380 of 8791; P = .71) remained unchanged. Decline in CFR after the mandate was associated with an increasing percentage of casualties transported in 60 minutes or less (regression coefficient, –0.141; P < .001), with projected vs actual CFR equating to 359 lives saved. Among 4542 casualties (mean injury severity score, 17.3; mortality, 10.1% 457 of 4542) with detailed data, there was a decrease in median transport time after the mandate (90 min vs 43 min; P < .001) and an increase in missions achieving prehospital helicopter transport in 60 minutes or less (24.8% 181 of 731 vs 75.2% 2867 of 3811; P < .001). When adjusted for injury severity score and time period, the percentage killed in action was lower for those critically injured who received a blood transfusion (6.8% 40 of 589 vs 51.0% 249 of 488; P < .001) and were transported in 60 minutes or less (25.7% 205 of 799 vs 30.2% 84 of 278; P < .01), while the percentage died of wounds was lower among those critically injured initially treated by combat support hospitals (9.1% 48 of 530 vs 15.7% 86 of 547; P < .01). Acute morbidity was higher among those critically injured who were transported in 60 minutes or less (36.9% 295 of 799 vs 27.3% 76 of 278; P < .01), those severely and critically injured initially treated at combat support hospitals (severely injured, 51.1% 161 of 315 vs 33.1% 104 of 314; P < .001; and critically injured, 39.8% 211 of 530 vs 29.3% 160 of 547; P < .001), and casualties who received a blood transfusion (50.2% 618 of 1231 vs 3.7% 121 of 3311; P < .001), emphasizing the need for timely advanced treatment. CONCLUSIONS AND RELEVANCE: A mandate made in 2009 by Secretary of Defense Gates reduced the time between combat injury and receiving definitive care. Prehospital transport time and treatment capability are important factors for casualty survival on the battlefield.